1. Field
The presently disclosed subject matter relates to a method for distance measurement and a distance measurement apparatus using the method for distance measurement. The distance measurement apparatus and method can be used whenever it is desired to measure a distance of a moving object with high accuracy and under various circumstances by emitting a modulated light towards the moving object. The disclosed subject matter is particularly suitable for use in a vehicle, a robot, security system and the like.
2. Description of the Related Art
A conventional method for distance measurement that can be employed in a vehicle and the like is disclosed, for example, in patent document No. 1 (Japanese Patent Application Laid Open JP2005-182137), patent document No. 2 (Japanese Patent Application Laid Open JP2000-75030), etc. The disclosed conventional methods include: emitting light using extremely high frequency (EHF) radar, a laser radar, etc., towards an object; detecting the reflex light that is dashed against the object; and calculating the distance by measuring a time from emitting the light to detecting the reflex light. The above disclosed conventional method is well known as “Time of Flight” (TOF) method.
However, when a laser light source is used as a light source in a distance measurement apparatus, a major driver is used to create an oscillation thereof. In addition, an output characteristic thereof should also be adjusted in order to prevent people that may be located in a direction of the distance measurement from being affected by the laser light source. Thus, the configuration including the major driver and the adjustor may result in a complicated distance measurement apparatus.
Therefore, another conventional method using the TOF method is disclosed, for example, in patent document No. 3 (Japanese Patent Application Laid Open JP2004-32682). According to this conventional method using the TOF method, a semiconductor light-emitting device such as an LED can be used as a light source and an image sensor including a plurality of photoelectric conversion chips can be used as a detector.
This disclosed conventional method includes: emitting a modulated light with a predetermined frequency towards an object using a semiconductor light-emitting device; detecting the reflex light dashed against the object; converting the reflex light into a charge using the image sensor; and calculating the distance by measuring a phase difference between the modulated light emitted from the light source and the reflex light reflected from the object.
However, according to a distance measurement apparatus using this disclosed conventional method, because an amount of light emitted from the light source is maintained at a constant level, the apparatus may have a negative impact on measurement accuracy of distance data. For instance, when the modulated light is too dark, because the ratio of the modulated light to a background light decreases, the signal to noise ratio decreases and therefore an error of measurement distance may increase. On the other hand, when the modulation light of the light source is too bright, because the image sensor is saturated with charges from the incoming light including the reflex light reflected from the object, the distance may not be measured.
In addition, because brightness decreases in inverse proportion to a square of the distance from the light source to an object, when the distance from the light source to an object is relatively near, such as when indoors, and a distance area measured is limited to the distance measurement apparatus, a large error may not be caused even if an amount of light emitted from the light source is maintained constant.
However, when the above distance measurement apparatus measures a distance from the driven vehicle to a vehicle in front thereof (i.e., when used outdoors), the objects may be varied such as a road, an oncoming car, a car traveling ahead, a guardrail, a pedestrian, etc., and measuring a distance from the distance measurement apparatus thereto may vary from several meters to infinity. Therefore, because a favorable amount of modulated light of the light source for measuring an object that is near to the distance measurement apparatus may be different from the amount of modulated light for measuring an object that is far from the distance measurement apparatus, the amount of modulated light of the light source should be dynamically changed in order to improve a measurement accuracy of distance according to a measuring distance, a background light and the like.
Furthermore, even if measuring various objects that are located at the same respective distances, the respective intensity of reflex light may change in accordance with the reflectivity of each of the measured objects. Thus, when measuring a distance of an object with a high degree of accuracy, the amount of modulated light should be changed according to a measuring condition and the like.
The above-referenced Patent Documents are listed below and are incorporated herein by reference.    1. Patent document No. 1: Japanese Patent Application Laid Open JP2005-182137    2. Patent document No. 2: Japanese Patent Application Laid Open JP2000-75030    3. Patent document No. 3: Japanese Patent Application Laid Open JP2004-32682 and its related patent family document, U.S. Pat. No. 7,119,350 to Hashimoto et al.